Mocha - How to test for unsettled promise?

I am testing a function that returns a promise. I want to assert that, in certain conditions, the returned promise would never settle (doesn't resolve nor reject).

How can I test this with Mocha?

---

If I run the following:

describe('under certain conditions', function () {
  let promise;
  beforeEach(function () {
    promise = new Promise((resolve, reject) => {});
  });
  it('should hang forever', function () {
    return promise;
  });
});

I get the following error:

Error: Timeout of 2000ms exceeded. For async tests and hooks, ensure "done()" is called; if returning a Promise, ensure it resolves